The present application is related to co-pending, commonly assigned application entitled xe2x80x9cLow Profile Overhead Projector Having a Foldable Postxe2x80x9d which was filed on the same day and which is hereby incorporated by reference.
The present invention relates to an improved optical design for an overhead projector. In particular, the present invention relates to an overhead Projector having a singlet lens optical design for an inverted projection head that reduces the height profile of the overhead projector and allows better sightlines for the audience.
A conventional overhead projector includes a light source, a horizontal light transmissive stage, and a projection head suspended above the stage. The stage includes a Fresnel lens. As illustrated in FIG. 1, in a conventional overhead projector using a single element meniscus projection lens, light passes from the light source through the Fresnel lens below the stage on which a transparent object containing the image to be projected is located. After the light passes through the transparent object, the light passes first through the single element meniscus projection lens and then is reflected off a folding mirror toward a projection screen. To obtain good image quality, the convex surface of the meniscus projection lens must face the stage, and the light from the projection lamp focuses at a stop position at a specified distance from the concave surface of the meniscus projection lens. The projection lens acts somewhat like a camera rear landscape lens, e.g., as described in Lens Design Fundamentals, by Rudolph Kingslake, relevant portions of which are hereby incorporated by reference, with the Fresnel lens focal distance defining the effective stop position.
The focal length of the projection lens and the distance to the projection screen determine the distance necessary between the stage and the lens (and therefore the height of the projection head) to create a focused image. A typical projection head height is approximately 420 mm, for a 292 mm focal length projection lens at image magnification of about 6xc3x97.
A protruding tall projection head tends to block the audience""s view of the screen. As a result, attempts have been made to lower the height of the projection head.
An alternative projection design is called an xe2x80x9cinverted headxe2x80x9d projector. As illustrated in FIG. 2a, in an inverted head projector, the projection lens is located such that light passes through the transparent object, then is reflected off a folding mirror and then passes through the projection lens and toward the projection screen. The mirror folds the light path, allowing the projection head assembly (which includes the folding mirror and the projection lens) to be positioned closer to the stage than with a non-inverted head assembly. However, since the mirror truncates the light cone closer to the base (and farther way from the waist of the light cone), the mirror dimensions are larger than those needed for a conventional head projector. FIG. 2b illustrates exemplary dimensions of a mirror, for a 292 mm projection lens at an image magnification of about 6xc3x97 and a projection head height of about 320 mm.
As a result, while the projection head is positioned lower, the projection head is larger in size. A low-height large projection head may be as obtrusive as a higher, but smaller, projection head.
In addition, a single element meniscus projection lens, with the mirror positioned below the lens, e.g., inverted, places the stop position of the projector in front of the lens, facing the concave surface of the lens. While this provides good optical performance, this creates a xe2x80x9chot spotxe2x80x9d in the open space directly in front of the projection lens. In overhead projectors using high power lamps, an unprotected hot spot may reach temperatures sufficient to ignite combustible materials or potentially to injure a user.
The need remains for an overhead projector having an optical design that reduces the obstruction of the lines of sight of the audience.
The present invention is directed to an overhead projector comprising a transmissive stage including a Fresnel lens, and a projection head placed above the stage. The projection head includes a mirror and a single element meniscus projection lens. The Fresnel lens focal point defines a projection lens stop position. The Fresnel lens defines a light cone and the mirror is a low profile mirror having dimensions corresponding to the size of light cone at the mirror.
A light path is defined from the Fresnel lens to the projection head and onto a projection surface. The mirror is placed ahead of the projection lens along the light path and the stop position of the projection lens is between the projection lens and the mirror.
The optical design is arranged such that,
FFres=(FLens(MLens+1)MLensxe2x88x92SLD)(MFres+1),
where the Fresnel lens has a focal length FFres, and a magnification MFres, the projection lens has a focal length FLens and a magnification MLens, and SLD is the distance from the projection lens stop position to the projection lens.
The single element meniscus lens includes a convex side and a concave side. In the present novel arrangement, the concave side of the projection lens faces the mirror, while the convex side faces the screen. The term xe2x80x9cfacesxe2x80x9d is referenced upon the position along the optical path.
In alternative embodiments of the present invention,
xe2x80x83250 mmxe2x89xa6FLensxe2x89xa6400 mm.
In a first exemplary embodiment, the projection lens has a focal length of about 355 mm and the Fresnel lens has a focal length of about 175 mm. In a second embodiment, the projection lens has a focal length of about 292 mm and the Fresnel lens has a focal length of about 140 mm. In a third exemplary embodiment, the projection lens has a focal length of about 400 mm and the Fresnel lens has a focal length of about 200 mm.
The overhead projector may further comprise a light source, wherein the Fresnel lens forms a focus point, the focus point being located between the mirror and the projection lens. A protective encasement may be placed to limit access to the focus point.